Update README.md

Author: meihuisu

README.md

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 The San Jacinto Fault Zone Velocity Model
 
+This San Jacinto Fault Zone velocity model was obtained using seismic tomography methods based on P and S arrival times to develop Vp, Vs and Vp/Vs models. This model was developed using a Vp/Vs inversion method based on double-difference (DD) tomography, which was adopted for the body wave part of our joint inversion to simultaneously invert 3-D wave speed models and seismic event locations using both absolute and differential arrival times. These methods were applied to a southern California plate boundary region using a data set that includes 247 472 P- and 105 448 S-wave phase picks recorded at 139 stations from 5493 events augmented by 151 575 differential travel times for better constraining the structure in the source regions. This model also incorporated 30 377 Rayleigh wave group travel times with periods ranging from 3 to 12 s.
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+Fang, H., H. Yao, H. Zhang, C. Thurber, Y. Ben-Zion, and R. D. van der Hilst, 2019. Vp/Vs tomography in the southern California plate boundary region using body- and surface-wave traveltime data, Geophys. J. Int., 216, 609–620, doi: 10.1093/gji/ggy458.
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 ## Installation
 
 This package is intended to be installed as part of the UCVM framework,